Synthesis gas is prepared predominantly by gasification, i.e., by steam treatment of coal or heavy petroleum fractions, in the first-mentioned case by the reaction: EQU C+H.sub.2 O.fwdarw.CO+H.sub.2 ( 1)
accompanied, however, by side reactions such that carbon dioxide and a little methane are also formed. By the gasification of petroleum fractions the amount of hydrogen produced in the synthesis gas is higher. Some coal and petroleum gasification processes involve the formation of greater amounts of methane, other hydrocarbons, tar, etc. During gasification a small amount of oxygen is normally added in order to render the gasification self-supplying with heat.
By various reactions the synthesis gas may be converted predominantly into methane and in recent years such reactions have gained an ever-increasing importance, i.e., for preparing substitute natural gas (SNG), component of special gas transport systems and to increase the energy supply. These reactions include: EQU CO+3H.sub.2 .revreaction.CH.sub.4 +H.sub.2 O (2) EQU 2CO+2H.sub.2 .revreaction.CH.sub.4 +CO.sub.2. (3)
The carbon dioxide may, however, also be converted with hydrogen into methane: EQU CO.sub.2 +4H.sub.2 .revreaction.CH.sub.4 +2H.sub.2 O. (4)
The so-called "shift reaction" causes an equilibrium between carbon monoxide and carbon dioxide: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2. (5)
Methane may also be formed as a by-product in the Fischer-Tropsch synthesis (hereinafter referred to as the FT-synthesis): EQU 2nCO+(n+1)H.sub.2 .fwdarw.C.sub.n H.sub.2n+2 +nCO.sub.2 ( 6)
(paraffin reaction) EQU 2nCO+nH.sub.2 .fwdarw.C.sub.n H.sub.2n +nCO.sub.2 ( 7)
(olefin reaction),
and possibly also: EQU nCO+2nH.sub.2 .fwdarw.C.sub.n H.sub.2n +nH.sub.2 O (8)
(olefin reaction).
The FT-synthesis predominantly produces higher hydrocarbons and is especially employed for the preparation of motor fuel and other liquid fuels. By a suitable choice of catalyst and process conditions it may yield a rather high proportion of methane.
A good review of methanation processes and catalysts is given by Mills et al in Catalysis Review 8(2), pp. 159-210 (1973).
The best catalyst for the preparation of methane from carbon oxides and hydrogen according to reactions (2), (3), and (4) is nickel on a support which normally consists of one or more refractory oxides e.g., chromium oxide, .gamma.-alumina, magnesium oxide, or silica, or mixtures thereof. Nickel may be present as oxide but during the methanation process in the strongly reducing environment it is predominantly present as free metal. Nickel is still the most important catalyst for methane production but nickel catalysts have the drawback that they are exceedingly sensitive to sulphur poisoning. The feed gas for a nickel catalyzed methanation process must, to a very high degree, be freed from sulphur, specifically from gaseous sulphur compounds. In practice the sulphur content is kept below 0.1-0.01 ppm by vol., dependent on the content of H.sub.2 in the synthesis gas and the temperature at the inlet to the catalyst bed. The sulphur deposition on the catalyst decreases with decreasing value of the ratio H.sub.2 S/H.sub.2 and increasing temperature (see J. R. Rostrup-Nielsen, "Steam Reforming Catalysts", Teknisk Forlag, Copenhagen 1975). The specificity for methanetion formation also decreases strongly with increased sulphur poisoning (see J. R. Rostrup-Nielsen and Karsten Pedersen, J. Catal. 59, p. 395 1979) for which reason it is normally desired that the sulphur coverage should be below 10%. Since the feed materials from which the feed gas is prepared, i.e., gasified coal or heavy fuel oil, as a rule are strongly sulphur-containing the feed gas for the methanation reaction must be subjected to an expensive and time-consuming sulphur purification process. The majority of metals have been used as Fischer-Tropsch catalysts, either as such or as oxides or hydroxides or possibly in a surface-sulphided form, but all the known catalysts are sulphur sensitive to a higher or lesser degree. This especially holds true for the important methanation and FT catalysts which are based on iron, cobalt or ruthenium.
Karla Wencke showed (Freiburger Forschungsh, A151, pp. 11-29 (1960)) that molybdenum as the free metal or oxide catalyzed the methanation of a synthesis gas with CO and H.sub.2, that it was advantageous to operate in a fluid bed and that the activity of the Mo-based catalysts for methane production decreased when small amounts of sulphur compounds were present in the synthesis gas. Madon and Shaw state in a reviewing paper in Catal. Review-Sci. Eng. 15(1), pp. 69-106 (1977)) that FT-catalysts based on metallic, oxidic or surface-sulphided molybdenum are subject to reduced activity in the presence of H.sub.2 S in the synthesis gas but that the effect is temporary and reversible so that the initial activity of the catalyst returns when sulphur is removed from the feed gas stream. In this respect molybdenum is different from, for example, nickel and ruthenium based catalysts in which the poisoning can be considered definitive and lasting because of strong affinity of these catalysts for sulphur and because the chemisorbed sulphur is in equilibrium with very low concentrations of H.sub.2 S. Madon and Shaw also call attention to the fact that a catalyst based on molybdenum sulphide is strongly selective for methane formation (more than 90% of the carbon present in the feed converted into hydrocarbons is converted into methane) but that the presence of larger amounts of H.sub.2 S in the feed gas causes a shift so that almost 30% are converted into C.sub.3-4 -hydrocarbons and only about 60% into methane.
From South African patent specification No. 766,137 it is known that, i.e., thoria, zirconia, hafnia and titania are FT-catalysts and comparatively sulphur-resistant. However, their catalytic activity is low and moreover they are selective to a considerable degree for forming aromatic and other higher hydrocarbons. The specification first and foremost is concerned with the use of vanadium based catalysts for making methane and test results with various forms of vanadium are set forth. It was found that a pre-sulphided catalyst of V.sub.2 O.sub.5 on a zeolite support has higher activity and selectivity for the formation of methane in the presence of up to 2% by volume of H.sub.2 S in the feed gas than without. Similar results were obtained with pure vanadium oxide (without support) and a similar yet improved activity was obtained with a high concentration of vanadium oxide on a support of alumina.
However, the activity and specificity of vanadium catalysts for methanation are not very high and in order to obtain a reasonably satisfactory activity a high concentration of vanadium on the catalyst is necessary, whether as oxide or sulphide. Correspondingly, known molybdenum catalysts are not satisfactory for methane production, partly because the activity is not satisfactory, partly because the activity usually decreases in the presence of sulphur, and particularly because known molybdenum catalysts favor higher hydrocarbons, notably C.sub.3-4 -hydrocarbons, at the expense of methane.
Vanadium and molybdenum based catalysts are known for various other purposes. Thus, U.S. Pat. No. 2,605,238 discloses a catalyst composition for use in vaporphase processes for the partial oxidation of organic compounds, e.g., for the manufacture of maleic anhydride from butylene. The catalyst consists essentially of a molybdenum trioxide and amorphous titanium dioxide. U.S. Pat. No. 3,464,930 discloses a catalyst for the gas phase oxidation of aromatic or unsaturated aliphatic hydrocarbons into carboxylic acid. The catalysts consist of an inert non-porous carrier coated with a mixture of vanadium pentoxide and titanium dioxide. U.S. Pat. No. 3,565,829 discloses a supported catalyst for oxidation reactions, e.g., the oxidation of o-xylene into phthalic anhydride. The catalyst comprises a non-porous support material and thereon a thin layer of an active composition consisting of a mixture of vanadium pentoxide, titanium dioxide and at least one oxide of aluminum, lithium and/or zirconium. German published patent application No. 24 36 009 discloses a supported catalyst for the oxidation of o-xylene or naphthalene into phthalic anhydride, comprising an inert, non-porous support with a thin coating of vanadium pentoxide, titanium dioxide and rubidium and/or cesium.
None of these methods and catalysts are usable for methane production and hence there still exists a need for providing a synthesis gas conversion which utilizes a catalyst which is fully sulphur-resistant, which is highly selective for producing methan and which has a high activity.